New 3D printing approach rises from the vat

18 January 2019

This shows a block M printed using the new vat-based approach developed by researchers at the University of Michigan. Photo: Evan Dougherty.

Rather than building up plastic filaments layer-by-layer, a new approach to 3D printing developed by researchers at the University of Michigan (U-M) can lift complex shapes from a vat of liquid up to 100 times faster than conventional 3D printing processes.

3D printing could change the game for small manufacturing jobs that produce fewer than 10,000 identical items, because it would mean that the objects could be made without the need for a mold that can cost upwards of $10,000. But the most familiar form of 3D printing, which involves building up 3D objects by depositing successive filaments or layers of a material in a defined pattern, hasn't been able to fill that gap on typical production timescales of a week or two.

"Using conventional approaches, that's not really attainable unless you have hundreds of machines," said Timothy Scott, U-M associate professor of chemical engineering who co-led the development of the new 3D printing approach with Mark Burns, professor of chemical engineering and biomedical engineering at U-M.

Their new approach, described in a paper in Science Advances, works by solidifying a liquid resin using two lights to control where the resin hardens – and where it stays fluid – which allows the team to solidify the resin in sophisticated patterns. They can make a 3D bas-relief in a single shot rather than by depositing a series of filaments or layers. Their printing demonstrations include a lattice, a toy boat and a block M.

"It's one of the first true 3D printers ever made," said Burns. But this true 3D approach is no mere stunt – it required overcoming the limitations of earlier vat-printing efforts. Namely, the tendency of the resin to solidify on the window that the light shines through, stopping the print job just as it gets started.

A previous solution to this solidification-on-window problem was a window that lets oxygen through. The oxygen penetrates into the resin and halts the solidification near the window, leaving a film of fluid that allows the newly printed surface to be pulled away.

But because this film is only about as thick as a piece of transparent tape, the resin must be very runny to flow fast enough into the tiny gap between the newly solidified object and the window as the part is pulled up. This has limited vat printing to small, customized products that will be treated relatively gently, such as dental devices and shoe insoles.

By replacing the oxygen with a second light to halt solidification, the Michigan team have been able to produce a much larger gap between the object and the window – millimeters thick – allowing resin to flow in thousands of times faster.

The key to their success is the chemistry of the resin. In conventional systems, there is only one reaction: a photoactivator hardens the resin wherever light shines. The Michigan system, by contrast, also employs a photoinhibitor that responds to a different wavelength of light.

By creating a relatively large region where no solidification occurs, this approach allows thicker resins – potentially with strengthening powder additives – to be used to produce more durable objects. It also bests the structural integrity of filament 3D printing, as those objects have weak points at the interfaces between layers.

"You can get much tougher, much more wear-resistant materials," Scott said.

In addition, rather than merely controlling solidification in a 2D plane, as current vat-printing techniques do, the Michigan team can pattern the two kinds of light to harden the resin at essentially any 3D place near the illumination window.

U-M has filed three patent applications to protect the multiple inventive aspects of this approach and Scott is preparing to launch a start-up company.

This story is adapted from material from the University of Michigan, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.

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